1. Field of the Invention
The present invention relates to an image pickup lens used for an image pickup apparatus incorporated in a portable terminal device or the like, and more particularly to a filter to be disposed concentrically with an optical axis of an imaging lens to exhibit an apodization effect or a peripheral light intensity correction effect.
2. Description of the Related Art
Generally, the performance of image pickup lenses is evaluated by the imaging capability at the focal plane. Consequently, a number of proposals with respect to aberration corrections for improving the imaging capability have been made. In addition to the imaging capability, it is also very important for image pickup lenses used in cameras, appearances of out-of-focus images (foreground blur and background blur). For example, in a portrait photograph with a person arranged in the center or a photograph of a flower obtained by macro photography, the impression of the photograph image depends largely on the appearance of an out-of-focus image of a portion other than the main subject (e.g., background portion). In macro photography of a subject using a macro lens with an imaging magnification of about ¼ to ½, in particular, a background portion other than the main subject portion, in most cases, becomes an out-of-focus image and, therefore, the appearance of the out-of-focus image is particularly important.
Various types of appearances of out-of-focus images are known and it is generally said to be a preferable appearance that gives soft atmosphere in which an original shape of an object in an out-of-focus image is easily identifiable with less muddy color mixture and is fluffily blurred as a whole.
As for a lens system designed for improving the appearance of an out-of-focus image, an image pickup lens having an apodization filter that exhibits an appodization effect is known as described, for example, in Japanese Unexamined Patent Publication Nos. 2006-145831 and 11(1999)-231209. The appodization effect improves an appearance of an out-of-focus image to the aforementioned favorable appearance by gradually decreasing the light transmission rate as the distance from the center of the optical axis is increased in a direction perpendicular to the optical axis and changing a light intensity distribution of the out-of-focus image.
Further, a peripheral light intensity correction filter is also known in which, contrary to the apodization filter, the light transmission rate is gradually increased as the distance from the center of the optical axis is increased in a direction perpendicular to the optical axis and equalizing light intensities in a central portion and a peripheral portion on the image forming plane.
When decreasing or increasing the light transmission rate according to the distance from the center of the optical axis in a direction perpendicular to the optical axis, the apodization filter or the peripheral light intensity correction filter can not efficiently exhibit the apodization effect or the peripheral light intensity correction effect unless the light transmission rate is changed accurately according to a predetermined rule.
An apodization filter or a peripheral light intensity correction filter is realized by printing or applying opaque dots or opaque concentric circles on a transparent substrate. Such filters change the light transmission rate by locally changing the application density of the dots or concentric circles. Conventional filters have difficulties in accurately changing the light transmission rate due to problems of accuracy in arranging a pattern of dots or concentric circles and in printing or applying the pattern. Such problems are more significant, in particular, for filters used in small image pickup apparatuses incorporated in portable terminals and the like.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a filter, an image pickup lens, an image pickup apparatus, and a portable terminal device capable of accurately controlling light transmission rate variation characteristics of the filter and effectively providing an apodization effect or a peripheral light intensity correction effect.